A personal pronoun must be of the same number, gender and person as the noun it stands for.
John is a good boy. He loves and respects his parents and teachers. (Here the pronoun he is of the same number, gender and person as the noun John.)
Alice is my sister. She lives abroad. (Here the pronoun she is of the same number, gender and person as the noun Alice.)
The students worked hard. Their teacher praised them. (Here the plural pronoun them is of the same number, gender and person as the plural noun students.)
A personal pronoun referring back to a collective noun must be in the singular number if the collective noun is viewed as a whole.
The jury has given its verdict.
The fleet will reach its destination in a week.
If the focus is on the individual members of the group, the pronoun should be in the plural number.
The jury were divided in their opinion.
When two or more singular nouns are joined by and, the pronoun used for them must be plural in number.
Both John and Peter played well. They were praised by the coach.
When two singular nouns joined by and refer to the same person, the pronoun must be singular in number.
The Secretary and Treasurer is negligent of his duty. (Here the nouns Secretary and Treasurer refer to the same person. Therefore we use a singular pronoun to refer back to them.)
Note that when two nouns refer to the same person, the determiner (the, my, this etc.) is used only before the first one.
My uncle and guardian is a doctor. (Here the nouns uncle and guardian refer to the same person.)
My uncle and my guardian are doctors. (Here the nouns uncle and guardian refer to two persons.)
When two singular nouns joined by and are preceded by each or every, the pronoun must be singular in number.
Every soldier and every sailor was in his place.
When two or more singular nouns are joined by or, either …or or neither…nor, the pronoun should be singular in number.
Either Alice or Mary forgot to take her umbrella.
Neither Peter nor John has done his homework.